Let's stick together
Generally, there are four types of adhesives used in bookbinding: pastes, dispersions, hide glues and hot melts. The two adhesives most widely used in hand bookbinding are starch paste and polyvinyl acetate (PVAc).
The strength of the adhesive is determined by abilities of its molecules to bond to the substrate, bond to themselves, and/or create mechanical interlocking with the substrate.
Starch Pastes
The use of starch paste goes back at least 2 millennia. Powdered starches are made from globules which during cooking release their two components amylose and amylopectin. It is the amylose which is responsible for the strength of the bond and different starches contain different percentages of amylose. Wheat starch contains a large amount of amylose (18-20%) and is considered stronger than other starches such as rice, corn or tapioca. Flour contains starch but also contains other components that interfere with the bonding.
A starch paste provides a surprisingly strong bond. One of the greatest advantages is a long open time to tack, allowing you to reposition materials without lifting the fibres of paper.
Starches are neutral, dry fairly clear, don’t degrade with time and are reversible. Once prepared, they should be stored in a cool location. The prepared shelf life is limited due to biological activity, so it is best to use a sealed container to delay the growth of mould. Storing it in a refrigerator will reduce the bonding strength.
The methylcellulose-group pastes, although not starch, act very much like a starch paste but provide a weaker bond. There is a large range of these products, having slightly different names, differing viscosities, soluble in specific liquids and prepared at differing temperatures. They work well for weak bonding requirements or as a mix into another adhesive.
Dispersion Adhesive
In a dispersion adhesive, the molecules that bind the two surfaces together are separated by a solvent. Once that solvent evaporates out, the molecules are allowed to link and form a strong bond between the two surfaces. In the case of polyvinyl acetate (PVAc), the solvent is water.
A good quality PVAc adhesive will dry clear, be close to neutral and have some additives for preservation in the container and plasticizers to help with flexibility. Not all PVAc adhesives or “white glues” have these qualities. During the drying process, there will be some off-gassing of acetic acid but once dry the PVAc adhesive will be near neutral. For this reason, the glued piece should not be immediately placed in a air tight container. If deterioration of the adhesive is allowed to occur, there can be more acetic acid off-gassing.
Another but not widely seen dispersion adhesive is polyvinyl alcohol (PVAi/PVOH). Because of the related term PVA, there can be confusion about the actual product. This adhesive is derived from PVAc through a hydrolysis process. The resulting product is mostly found as a powder and must be cooked in a process similiar to the starch pastes. It has excellent qualities such as neutral, dries clear, remains flexible over time and is reversible with water.
A third but not widely seen is ethylene-vinyl acetate (EVA). EVA remains very flexible but will off-gas acetic acid like PVAc. It is reversible. Many hot-melts are made from the solid form of this product.
Dispersion adhesives usually have a shorter open time to tack but by mixing with a starch or CMC-group paste, this time can be extended. The added paste provides an increased source of water without compromising the viscosity of the adhesive.
Animal Glue
Animal glues (hide glue, gelatine glue, fish glue) were traditionally used but aren’t considered ideal anymore except or maybe duplicating materials in a historic binding. These adhesives provided food for insects and become brittle with age. They do have the advantage of quick set time and are reversible with water. Heat will allow you to reposition materials.
Hot-melt
Holt-melts are now very common in the commercial bookbinding industry but aren’t practical for the hand bookbinder.
Freezing of Pastes & Dispersions
When water is the solvent and it freezes into ice crystals, the adhesive molecules are now allowed to form links with themselves which are irreversible when the ice crystals melt. The adhesives become jelly-like masses. This is the reason these adhesives cannot be shipped in the winter months where there is a chance of freezing during the transport.